A faraday cage is mainly used for shielding against electric fields by being a gaussian shell (albeit with holes) inside of which no electric fields shall reside (well mostly). It definitely means that your cellphone would not receive a signal (try calling it) while inside a closed stainless steel tiffin box but 50Hz magnetic fields from transformers may not be blocked by it.
Magnetic fields are best blocked nearest to their source (transformer etc.) by closing a path using high permeability material like electrical steel etc. A good example of this is the shielding pot used in a loudspeaker with a neodymium magnet.
Magnetic fields are best blocked nearest to their source (transformer etc.) by closing a path using high permeability material like electrical steel etc. A good example of this is the shielding pot used in a loudspeaker with a neodymium magnet.
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Its called a GOSS band because most often they use a thin strip of Grain Oriented Silicon Steel. The GOSS forms an electrically closed band around the outside of the transformer and is in effect a Gauss band capturing any stray fields from the transformer via a shorted turn.
5. MINIMISATION OF STRAY FIELD
A copper screen can be included during manufacture resulting in an 80% reduction in stray field. Thus reducing electron magnetic interference and opportunities for flux to escape. A metal strip of audio grade steel (GOSS Band) can further be wound around the finished transformer keeping the radiated magnetic field to a minimum.
Source: https://trans-tronic.co.uk/toroid-vs-laminated/
Source: Talema Transformers
The transformers I use have a single band of GOSS around them and the band is electrically conductive - so its not copper and then steel - its just steel.
You may have more info/different info or better info, but this is how I understand it.
🙂
5. MINIMISATION OF STRAY FIELD
A copper screen can be included during manufacture resulting in an 80% reduction in stray field. Thus reducing electron magnetic interference and opportunities for flux to escape. A metal strip of audio grade steel (GOSS Band) can further be wound around the finished transformer keeping the radiated magnetic field to a minimum.
Source: https://trans-tronic.co.uk/toroid-vs-laminated/
Source: Talema Transformers
The transformers I use have a single band of GOSS around them and the band is electrically conductive - so its not copper and then steel - its just steel.
You may have more info/different info or better info, but this is how I understand it.
🙂
There are also transformers with an outside copper band. Its acts as a shorted turn to leakage flux, but not to pri/sec coupled flux.
The GOSS band is a commercial denomination, and it is essentially a permeability-based magnetic shield (comparable to mumetal). This means that it operates down to DC (static fields), unlike a Gauss band, which is a shorted turn and only operates for varying fields (AC).
The orientation is different too: a GOSS for a toroidal is annular, and follows the circumference, but a suitable Gauss band for a toroid would need to encompass the whole volume in a conductive enclosure, a kind of flattened sphere.
In principle, properly wound toroidals should not require any shield at all: an ideal toroid has a zero external field, but it requires demanding conditions: perfectly homogeneous and regular primary and secondary windings, and compulsory even-numbered of layers for both, which is extremely complicated to achieve in practice (just scramble-winding an ordinary toroid is already complicated enough)
The orientation is different too: a GOSS for a toroidal is annular, and follows the circumference, but a suitable Gauss band for a toroid would need to encompass the whole volume in a conductive enclosure, a kind of flattened sphere.
In principle, properly wound toroidals should not require any shield at all: an ideal toroid has a zero external field, but it requires demanding conditions: perfectly homogeneous and regular primary and secondary windings, and compulsory even-numbered of layers for both, which is extremely complicated to achieve in practice (just scramble-winding an ordinary toroid is already complicated enough)
I have only ever used a GOSS band on power transformer. I guess the copper Gauss shield, or ‘enclosure’ would really be something that operates at HF?
Yes, you could see the shorting (copper) around the air-gaps of flyback (ferrite HF) transformers, that are notorious for having high levels of fringing flux.
http://www.e-magnetica.pl/doku.php/flux_band
http://www.e-magnetica.pl/doku.php/flux_band
From "Electromagnetic Compatibility Engineering" (2009), page 526:
The easiest way of reducing the leakage flux from an E-core transformer is by the use of a shorted turn (sometimes called a ‘‘belly band’’). A shorted turn is a wide band of copper that wraps around the transformer’s windings (see Nave, 1991, p. 180). The shorted turn is coupled magnetically only to the leakage flux, and it acts as a low-impedance high-current secondary winding. The current induced into the shorted turn produces a magnetic field, which is opposite in polarity to the leakage flux and thereby cancels most of the original leakage.
The easiest way of reducing the leakage flux from an E-core transformer is by the use of a shorted turn (sometimes called a ‘‘belly band’’). A shorted turn is a wide band of copper that wraps around the transformer’s windings (see Nave, 1991, p. 180). The shorted turn is coupled magnetically only to the leakage flux, and it acts as a low-impedance high-current secondary winding. The current induced into the shorted turn produces a magnetic field, which is opposite in polarity to the leakage flux and thereby cancels most of the original leakage.
Not only: it depends on the thickness.
Gauss bands for 50/60Hz transformers are pretty substantial, but they operate mainly for the odd harmonics, because that's what most of the leakage flux consists of
For toroidal transformers I use low frequency high permeability mu metal 1/8” thick, one wrap will absorb about 75% of magnetic fields, then using a spacer like cotton between the two mu metal layers, wrap a second time, two layers absorb about 90% of mag field.
Thanks for all the information here. I have a large balanced power transformer that I'd like to increase the shielding of and this is really useful.
@geoffkait I can only find really thin sheets of mu metal, nowhere near 1/8" thick. Would you mind posting a link to your source so I can check it out please?
many thanks,
Crom
@geoffkait I can only find really thin sheets of mu metal, nowhere near 1/8" thick. Would you mind posting a link to your source so I can check it out please?
many thanks,
Crom
It’s been quite a while but I bought my mu metal on eBay. It may have been only 1/16” thick. I just saw some Ultraperm mu metal on eBay, maybe it’s a standard thickness.
If you are stuck to use inductors in your audio path, I suggest to use double inductors in an arrangement that cancels external fields.
This works for decades in magnetic "humbucker" pickups used with electric guitars.
This works for decades in magnetic "humbucker" pickups used with electric guitars.
That explains a lot! I used it once, bending cutting and shaping as I went with almost no improvement.
There are actually many reasons why someone might not get good results with mu metal including but not limited to: they installed the wrong type of mu metal, their ears aren’t as good as they think they are, some transformers are less affected by mu metal than others and/or the system is not revealing enough. Like anything else in this hobby. 😬
”Almost no improvement?” You mean there was some improvement? 🤗
”Almost no improvement?” You mean there was some improvement? 🤗
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Properly annealed mu metal can have permeability 100 or so times higher than ordinary steel, which is why its so effective if used well. The slightest deformation will drastically reduce the permeability as it destroys the large grain size of the microstructure and stacks up dislocations - this means the magnetic domains get way smaller leading to low permeability. Handle it like its glass...
Uh, mu metal is already deformed to some extent during manufacture, like any metal that is rolled. the deformation that occurs when wrapping and bending only occurs for a small percentage of the total area of the mu metal, so can be ignored..
It is annealed after that. And annealed again after working it. Some places have the equipment and the expertise to do it properly. Professional audio transformer manufacturers tend to have the equipment. For example, Cinemag has a hydrogen annealing oven.
Sounds like there will soon be a spin-off thread 'How to build a DIY hydrogen annealing oven' in the Equipment & Tools subforum 😉 I guess the main problem is to ensure the hydrogen won't explode.